This invention relates to an image sensor which comprises a large number of photosensors arranged to form an array.
Various solid-state facsimile transmitters have recently been developed in which image sensors, e.g., MOS or CCD elements fabricated by IC techniques, cooperate with lenses to achieve high speed operation and high reliability. In conventional solid state facsimile transmitters, images of manuscripts to be transmitted are reduced in size by lenses for focusing them onto the image sensor, however, thereby creating a deficiency in that the internal paths of light effectively become longer which causes the transmitters to be larger. Additionally, high accuracy is required in such arrangements of lenses and sensors.
To eliminate these deficiencies, a contact type image sensor as shown in FIG. 1 has been developed. In the photosensor array of FIG. 1, light emitted from an array of light emitting diodes 1 is reflected by a manuscript 2 and transmitted through a rod lens 3 to an array of amorphous silicon 5 (hereinafter referred to as "a-Si") formed on a base plate 4. The array of light emitting diodes (LEDs) 1 emits green light to which a-Si has high sensitivity. The manuscript 2 and a-Si sensor 5, which coact by placing rod lens 3 between them, are of the same size. Since the device requires no focusing lens, the distance between sensor 5 and manuscript 2 is reduced to less than 10 millimeters, as compared with the 600 millimeter spacing which was the case in the conventional device described previously. This provides a small image sensor for facsimile purposes.
Various materials, such as glass or ceramic, may be used as base plate 4, and a-Si sensor 5 is formed by glow discharge deposition from gassy silan. The deposited a-Si layer is patterned by using conventional photo etching techniques to form eight to sixteen photosensors per millimeter. Other integrated circuits may be formed on the same base plate 4 and connected to the a-Si photosensors.
FIG. 2 shows an equivalent electrical circuit diagram of the a-Si image sensor of FIG. 1. In the circuit, one thousand seven hundred twenty eight (1728) pairs of elements, each pair consisting of a photosensor 6 and a blocking diode 7, are connected in series to form a sensor array for reading manuscripts of conventional A4 size. The pairs are divided into thirty two (32) blocks of fifty four (54) pairs each. One terminal of each photosensor 6 is connected to a cathode of a corresponding blocking diode 7, respectively, and the other terminal of all fifty four photosensors in each block are connected together and to a respective switch 8. The corresponding anodes of the fifty four blocking diodes in each block are connected to a respective one of fifty four common leads which are, in turn, connected to one of fifty four switches 9, respectively. All the common leads are connected together through switches 9 to a positive terminal of a voltage source 11 via an output resistor 10. In the circuit of FIG. 2, threfore, the 1728 photosensors 6 are consecutively scanned by sequentially operating driving switches 8 and 9 to provide an output signal between output terminals 12 and 13.
A cross-sectional view of the photosensors of FIG. 1 is shown in FIG. 3. Each photosensor comprises a glass base plate 14, a metal electrode 15 composed of two layers of Pt and Ni, and a transparent electrode 16 of ITO, SnO.sub.2, etc. Each electrode 15, 16 is formed on the base plate 14; an a-Si layer 17 is formed on electrodes 15 and 16; and a metal electrode 18 of Al or the like is formed on a-Si layer 17. A metal electrode 19 provided on glass base plate 14 is connected to transparent electrode 16.
The a-Si layer 17 comprises an i-layer 20 about 1 .mu.m thick, and an n-layer 21 about 500 .ANG. thick, as shown in FIG. 4. Electrode 16, a-Si layer 17 and upper electrode 18 together form a photosensitive portion. Electrode 19 connected to transparent electrode 16 is employed as a lead for connecting the photosensor to switch 8, as schematically depcited in FIG. 2. Metal electrode 15 is connected to switch 9 together with the corresponding electrodes of the other blocks. The arrow marked as reference character 22 depicts the direction from which light strikes and ultimately passes comes through glass base plate 14. A blocking diode is formed at the region where a-Si layer 17 is sandwiched by metal electrodes 15 and 18. Since the incident light 22 is reflected by the metal electrode 15 and does not reach the junction of the diode, the blocking diode operates as an ordinary diode, i.e., non-photosensitive. The junction of the blocking diode is formed at that portion where the Pt layer of electrode 15 contacts the i-layer 20 of a-Si layer 17, thus comprising a Schottky barrier junction.
Photosensor 6 is reverse biased in the absence of incident light. Light striking any of the photosensors is thus detected by selecting the respective sensor by driving switches 8 and 9. For example, when switches 81 and 91 located at the left hand side in FIG. 2 are closed, current from voltage source 11 flows through photosensor 61 at a magnitude dependent upon the amount of light coming into photosensor 61. The voltage drop across resistor 10 through which this current flows is thus read out as an output signal.
In the conventional device mentioned above, the following problems arise when any one of blocking diodes 7 is defective. If the defect is a short circuit between electrodes 16 and 18 in any of the blocking diodes, the corresponding sensor is always rendered conductive regardless of the existence of incident light; when scanned, such a sensor will always generate an output signal indicative of a "bright", i.e. illuminated, condition. Alternatively, if either of electrodes 16 and 18 is defective in its intended contact with a-Si layer 17, the signal read out when scanned will always be indicative of a "dark" condition. In the case of a short circuit existing in one of blocking diodes 7 such that the reverse current is not sufficiently blocked, the current from the corresponding photosensor is unimpeded and flows through output resistor 10 to disturb the current levels of the other photosensors. If any one of these defects exists, photosensor 5 as a whole is rendered defective so that the manufacturing yield of such contact-type photoreading devices decreases.
The conventional device has a further undesirable aspect. Although photosensors 6 and blocking diodes 7 are integrally formed with each other, as shown in FIG. 3, switches 8 and 9 are formed separately from the photosensor and blocking diodes. This separate formation adds to manufacturing costs and effort, as well as increasing the number of sites prone to failure during machine operation.